污染预防的具体做法可分几个层次？分别是什么？
参考答案：分三个层次。分别为源头控制、过程控制、末端治理。
换热器管束发生泄漏时，首先应关对装置生产影响小一侧的进出口阀门，改副线操作。
We ______ our dinner when a neighbor came to our house.
A．had just had
B．have just had
C．just had
D．just have had
